Internal imaging is often enhanced through the use of injected imaging agents that allow an imaging technique (MRI, CTA, x-ray, ultrasound, etc.) to provide a brighter, clearer image of one or more anatomic structures. Imaging agents are formulated for a particular type of imaging technology and are used to better detect and differentiate the targeted vessels or other tissues from surrounding structures, and particularly so when compared to images of such tissues acquired without the use of such contrast agents.
Many contrast agents are directed toward imaging of the vasculature system. These agents are injected into the blood stream, either by artery or vein, and delineate the presence or absence of blood in the human vasculature. This type of imaging presents problems not encountered during other types of internal imaging, such as those employed in orthopedic studies, where a simple needle is used to inject contrast agent into a static joint socket or the like.
For example, vascular imaging presents the added difficulty of capturing the image of a moving target, either moving contrast agent or moving body structures like the heart, for example. Because the imaging agent is injected directly into the flow of the blood stream, the agent is immediately carried away and mixed with the blood, thereby weakening the contrast concentration it provides. Hence, the agent is typically delivered through a long catheter threaded through the vasculature to a location proximate the target area. Delivery catheters typically have an open end from which the agent is injected. Because the blood is flowing in the same direction the agent is expelled from the catheter, the agent is carried away quickly, necessitating the injection of a large volume of agent in order to achieve the desired results. Alternatively, imaging sometimes involves injection of agents against the blood flow. In this situation, the agent is similarly carried away.
Some catheters attempt to mitigate this effect by providing openings in the sides of the catheter rather than just at the end. Quite often, however, these side openings are blocked because the catheter is resting along an inside wall of the vessel lumen. This can result in a diminished contrast effect and, due to the increased fluid pressure through the holes that are not blocked, trauma to the vessel. Furthermore, because of the high volume of agent being injected, the exit holes from the catheter are typically large, thereby reducing the amount of force necessary to inject the agent at a high flow rate. The large volume of agent being pushed through these holes adds to the potential vessel trauma in two ways. First, even if the catheter holes are not blocked on one side by the vessel walls, the jet effect of the high volume of contrast agent injection rapidly exiting the holes may injure tissue as the fluid streams impact the vessel walls. Second, the jet effect may cause the catheter to move or whip during injection, damaging tissue as the catheter itself impinges on the vessel walls.
One example of a needle or catheter that incorporates holes along its sides is shown and described in U.S. Pat. No. 6,855,132, issued Feb. 15, 2005 to Van Tassel, et al., entitled Apparatus With Weeping Tip And Method Of Use, and in U.S. Pat. No. 6,969,373, issued Nov. 29, 2005 to Schwartz, et al., entitled Syringe System. Both of these references are incorporated by reference herein. Some of the embodiments of catheters discussed in these references mitigate jet effect by providing large numbers of smaller holes, thereby reducing the fluid velocity through any given hole. However, the embodiments of catheters in these references do not address retrograde flow problems associated with ejecting fluid through side holes.
Retrograde flow occurs occasionally when ejecting contrast agent at high velocities through catheters with side holes. If the blood stream flow rate is significantly slower than the injectate flow rate, it is possible to encounter contrast agent traveling upstream. This diminishes the definition of the bolus image. During applications where agent is used for opacification of the heart, injectate entering the venous side of the heart has to travel through the pulmonary system before entering the arterial side of the heart. The pulmonary system significantly diffuses the bolus, making imaging of the arterial side more difficult. Hence, beginning with a highly concentrated, well defined bolus of contrast agent becomes paramount.